Mobile robots are being used more and more to perform tedious and dangerous tasks and to provide security. A serious problem is presented by autonomous mobile robots which patrol a secured space in some pattern or randomly. To keep from becoming lost, stymied and ineffective the robot must know where it is. In the past, robots have been made to home in on a signal to return to base. But this does not help determine position; it only vectors home the robot. If a fire or intrusion occurred the robot might detect it but would be unable to relay its location. Most robots cannot safely travel more than a few feet from the homing signal. Closer control can be achieved by connecting the robot by cable directly to a central computer. But the disadvantages of trailing a cable about make this approach unappealing. Typically the locator signals are infrared and suffer from high background noise due to ambient light such as daylight, incandescent and fluorescent. Because of this some systems are required to be operated in the dark or very low light conditions.